Germanium-on-silicon mid-infrared waveguides and Mach-Zehnder interferometers

Abstract

Abstract-In this paper we describe Ge-on-Si waveguides and Mach-Zehnder interferometers operating in the 5.2 -5.4 µm wavelength range. 3dB/cm waveguide losses and Mach-Zehnder interferometers with 20dB extinction ratio are presented. Index Terms-mid-infrared, photonic integrated circuits, germanium on silicon I. INTRODUCTIONThe mid-infrared wavelength range is a region of interest for spectroscopic sensing applications, due to strong and specific absorption features of many molecules in that wavelength range [1]. This allows for a non-contact method to detect the presence and concentration of these molecules. Quantum cascade lasers and inter-band cascade lasers are used as coherent laser sources in this wavelength range. These are typically single-wavelength lasers with limited tuning range. Other options are the use of broadband incoherent thermal sources or coherent supercontinuum sources in combination with a spectrometer. Current instrumentation is bulky, expensive and power hungry, necessitating the need for a midinfrared photonic integration platform to mitigate these disadvantages. Several functions can be implemented on such a platform, such as wavelength multiplexers for arrays of single wavelength QCL lasers allowing to cover a broad wavelength range and the integration of spectrometers for spectral analysis of broadband sources after they passed through a gas or liquid under study. For telecom wavelengths, Silicon-on-Insulator (SOI) is now a standard waveguide platform. One of the main reasons for this is the compatibility with CMOS fabrication technology and the associated potential for large volume manufacturing at low cost. However, beyond 3.8 µm, the underlying buried oxide layer starts absorbing heavily [2]. There are several candidates for viable waveguide platforms for the mid-IR wavelength range beyond 3.8 µm as discussed primarily in [2]. There have been reports in recent literature on Ge-on-Si waveguides [3], free-standing Si waveguides [4] and silicon-on-sapphire waveguides [5]. In this paper we describe the fabrication of Ge-on-Si photonic integrated circuits, an attractive option for mid-IR photonics and present waveguide losses and a key photonic component in wavelength multiplexers and spectrometers: the Mach-Zehnder Interferometer (MZI).

Silicon photonic platforms for mid-infrared applications [Invited]

“…Though the optical power at two output ports keeps an acceptable difference (∼0.76 dB), the insertion loss is too large (∼23 dB) for the device application and integration. [49,50]. The design approach of these commonly used components is easily transferred from those at the NIR.…”

“…With regard to the functionality, several MIR passive devices on different platforms have been demonstrated, including the fiber-chip grating couplers [5,[43][44][45][46], multimode interferometers (MMIs) [6,33,47,48], Mach-Zehnder interferometers (MZIs) [49,50], MRRs [8,9,12,14,17,18,[51][52][53], photonic crystals (PhCs) [54][55][56], de/multiplexers [10,[57][58][59][60], polarization splitter and rotator (PSR) [61], and polarization beam splitter (PBS) [62]. These pioneering works open the way to MIR photonic integration.…”

Modeling of second harmonic generation in hole-doped silicon-germanium quantum wells for mid-infrared sensing

“…The giant nonlinear response of MQWs and the strong absorption coefficient of ISBTs imply that a few hundred microns of interaction length are usually sufficient to deplete the pump beam and obtain maximum conversion efficiency [19]. Recently, state-of-the art MIR waveguides have been realized in both the Ge-on-Si and the SiGe-on-Si platforms, exploiting the difference in refractive indexes between Ge (n = 4.0) and Si (n = 3.4) in the MIR [39][40][41][42]. An example of a sensing configuration is shown in Fig.…”

Group IV mid-infrared photonics

“…In the MIR they can be particularly useful, since the self-imaging distance in an MMI is inversely proportional to the wavelength, so the V 2= I 006 pm device length can be very short. In the MIR, SOI MMIs have previously been demonstrated at 3.8 μm, and in Ge-on-Si they have been used as splitters/couplers in a Mach-Zehnder interferometer [5], although their loss was not reported. …”

“…Ge-on-Si waveguides have already been demonstrated with losses of 2.5-3.0 dB/cm at λ=5.8 µm by Chang et.al [3]. Since then Ge-on-Si multiplexers [4], Mach-Zehnder interferometers [5], and thermo-optic modulators [6] have also been investigated. We report a record low loss in Ge-on-Si and a first demonstration of all optical modulation in such waveguides.…”